Exhaust gas emitted from automobile and other internal combustion engines contains components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOX). Various types of catalyst technologies have been examined in order to purify these components.
A known example of a catalyst system for purifying exhaust gas emitted from diesel engines is a catalyst system that combines a diesel oxidation catalyst (DOC) device and a selective catalytic reduction (SCR) device. In this catalyst system, HC are purified by being oxidized to water (H2O) and carbon dioxide (CO2) by the DOC and CO is purified by being oxidized to carbon dioxide (CO2) by the DOC, while NOX (consisting mainly of nitrogen oxide (NO) and nitrogen dioxide (NO2)) are purified by being reduced to nitrogen (N2) using a reducing agent such as urea by the SCR after a portion of the NO is first oxidized to NO2 by the DOC. In this SCR, the rate of the NOX reduction reaction is known to be fast when there are equimolar amounts of NO and NO2.
Various studies have been conducted on the DOC used in this catalyst system in attempt to improve the oxidation efficiency of HC, CO and NO.
For example, the device for purifying diesel exhaust gas described in PTL 1 is provided with a zoned arrangement of catalysts having a first oxidation catalyst arranged on a support substrate as a front catalyst, and a second oxidation catalyst as a rear catalyst immediately thereafter arranged on this support substrate, in the direction of exhaust gas flow. The support base is a flow-through substrate, and the first and second oxidation catalysts contain palladium and platinum in the catalytically active coating thereof. The total amounts of platinum and palladium in the front catalyst are 30 g/ft3 to 250 g/ft3, respectively, which are higher than those of the rear catalyst of 5 g/ft3 to 100 g/ft3, and the Pt:Pd ratio in the front catalyst is lower than that of the rear catalyst.
PTL 2 describes an oxidation catalyst composite material for reducing exhaust gas emissions from lean-burning engines, which contains a support substrate having a length, an inlet end and an outlet end, and a catalyst material of an oxidation catalyst on this support; wherein, the catalyst material of the oxidation catalyst comprises a first washcoat containing zeolite, Pt and a first refractory Mn-containing metal oxide support, a second washcoat containing a second refractory metal oxide support and a platinum (Pt) component and palladium (Pd) component having a Pt:Pd ratio within the range of about 10:1 to 1:10, and a third washcoat containing palladium and a rare earth oxide component, but not substantially not containing platinum, and is effective for reducing hydrocarbons and carbon monoxide present in the exhaust gas of lean-burning engines and for oxidizing NO to NO2. It is explained in this PTL 2 that an arbitrary arrangement can be used for the arrangement of the first to third washcoats on the support substrate.